Eliciting antibodies that neutralize human immunodeficiency virus type-1 (HIV-1) by vaccination is a daunting challenge, due to features of the HIV-1 envelope glycoprotein (Env) that render it inherently resistant to antibody responses, and the extraordinary antigenic diversity that has accrued since the beginning of the HIV-1 pandemic. To circumvent these formidable obstacles, we are pursuing durable in vivo expression of a synthetic neutralizing antibody by gene therapy as an alternative to conventional vaccine approaches. This synthetic antibody (eCD4-Ig) is based on the Env-binding determinants of the viral receptor (CD4) and the viral coreceptor (CCR5). Consequently, eCD4-Ig neutralizes 100% of strains of HIV-1 and simian immunodeficiency virus (SIV) tested thus far, protects rhesus macaques from infection with simian-human immunodeficiency virus (SHIV), and is itself a poor target for host immune responses. Antibody functions in addition to virus neutralization, including antibody-dependent cell-mediated cytotoxicity (ADCC), may substantially augment the ability of eCD4-Ig to prevent systemic infection following exposure to HIV-1. The goal of this project is to maximize the capacity of eCD4-Ig to direct the elimination of virus-infected cells by ADCC. The resulting ADCC-optimized version of eCD4-Ig may confer greater protection against HIV-1 infection, reduce the concentration of eCD4-Ig that must be sustained in plasma and the mucosa to reliably prevent HIV-1 infection, and may increase its effectiveness as a therapeutic in people already infected with HIV-1.